A technique where a molten steel is cast directly into a thin strip-like slab having a thickness of not more than 10 mm has recently been developed and tested using actual equipment. This new technique can simplify or eliminate the step of hot rolling.
A slab having a thickness exceeding 100 mm has hitherto been hot-rolled by means of a hot-rolling mill, consuming much energy. Therefore, the simplification or elimination of the step of hot rolling has been desired from the viewpoint of lower production costs as well as of the environment. The process involving the step of casting a molten steel into a thin strip-like slab having a thickness of not more than 10 mm will be hereinafter referred to as the "new process," while a process involving the step of hot-rolling a slab to a thin strip-like slab will be hereinafter referred to as the "current process."
The production of a cold-rolled sheet of a Cr-Ni-base stainless steel, represented by a 18%Cr-8%Ni steel, or a Cr-base stainless steel by the new process has posed a problem of surface roughening (called "orange peel" or "roping") created on the surface of products.
For example, an article in ZAIRYO TO PUROSESU (Current Advances in Materials and Processes) Vol. 3 (1990), p. 770, published by The Iron and Steel Institute of Japan describes a phenomenon of deterioration in surface quality of sheet products of SUS304 produced by the new process. According to this article, orange peel-like surface roughening (roping) is described to be created on the surface of a cold-rolled steel sheet due to the presence of coarse grains in the material before finish cold rolling. In order to prevent this phenomenon, the refinement of grains of the material before the finish cold rolling by the following two means is described to be useful.
1) The thin strip-like slab is subjected to hot rolling and annealing of the hot-rolled sheet. For example, it is hot-rolled at 1200.degree. C. with a reduction ratio of 16% and subjected to solution treatment at 1150.degree. C. for one min. PA1 2) The thin strip-like slab is subjected to cold rolling twice with intermediate annealing being performed therebetween. For example, it is rolled at room temperature with a reduction ratio of 10% and subjected to intermediate annealing and then to finish rolling.
Further, an article in ZAIRYO TO PUROSESU (Current Advances in Materials and Processes) Vol. 4 (1991), p. 996, discloses that, although the surface roughening (roping) of a cold-rolled sheet can be alleviated by subjecting the cold-rolled sheet to temper rolling with a high reduction ratio, this technique deteriorates the quality, particularly the elongation, of the material, necessitating the regulation of the steel composition so as to be brought to a .gamma.-phase unstable composition system, i.e., high Md30. For example, it describes that roping and workability (elongation) can be made comparable with those in the case of the current process by bringing Md30 to 30.degree. C. and conducting temper rolling with a reduction ratio of 1%.
Furthermore, an article in ZAIRYO TO PUROSESU (Current Advances in Materials and Processes) Vol. 4 (1991), p. 997 describes that the surface roughening (roping) of a cold-rolled sheet can be alleviated by designing the constituents so as to increase the .delta.-ferrite content, thereby providing a .gamma.-phase unstable composition.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2-133528 describes that hot rolling, at a temperature of 900.degree. C. or above, with a reduction ratio of not more than 60% results in the recrystallization of the texture of the thin strip-like slab, alleviating roping. This technique is described to alleviate the roping independently of the practice of annealing after hot rolling. Further, regarding the heat history of the strip after hot rolling, the document describes only that the strip is cooled in the temperature range of 900 to 550.degree. C. at a rate of not less than 50.degree. C./sec.
Regarding the production of a sheet of a Cr-base stainless steel having reduced ridging by a thin strip-like slab casting process, Japanese Unexamined Patent Publication (Kokai) No. 2-166233 discloses a production process which comprises the steps of: casting a steel into a thin strip-like slab having a thickness of not more than 10 mm; subjecting the thin strip-like slab to rolling with a reduction ratio of not less than 20% at a temperature above the .gamma.-phase precipitation initiation temperature; immediately after the rolling, introducing the resultant steel sheet into a soaking pit and holding in the above temperature region for 3 sec to 5 min; and then subjecting the steel sheet to coiling, cold rolling, and annealing.
Further, in order to prevent ridging of a C-base stainless steel strip, Japanese Unexamined Patent Publication (Kokoku) No. 62-136526 discloses a process which comprises the steps of: casting a steel into a thin strip-like slab having a thickness of not more than 60 mm; holding the slab in the temperature range of 900 to 1150.degree. C. for 5 min or longer; hot-rolling the slab with final hot rolling being conducted at 800 to 1100.degree. C.; and then subjected to a series of conventional steps, i.e., annealing, picking, cold rolling, and finish rolling.
Further, with a view to improving the toughness of a sheet of Cr-base stainless steel, EP 0638653 A1 discloses a technique where a thin strip-like slab having a thickness of not more than 10 mm is hot-rolled in the temperature range of 1150 to 950.degree. C. with a reduction ratio of 5 to 40% and then passed through a heat-treating furnace, kept in that temperature region, over a period of 5 sec or longer, to permit carbonitride or the like contained in the rolled strip to be satisfactorily precipitated and grown.
In the above known publications, however, even when heat treatment is performed after hot rolling of the thin cast strip-like slab in order to improve the surface quality, no means for the heat treatment is specifically disclosed. Similarly, in the case of a technique where, after hot rolling, heat treatment is performed in order to improve the toughness, no means for the heat treatment is specifically disclosed.
Further, none of the above known publications disclose means for solving problems caused in direct rolling of the thin strip-like cast slab, such as meander, breaking, and creation of defects such as scab.
Furthermore, none of the above known publications propose means for alleviating the creation of roping due to uneven heat treatment caused in the case where the thickness of the thin cast strip-like slab is changed during casting.
An object of the present invention is to solve the above various problems caused in the production of sheet products using a continuous thin cast strip-like slab.
Another object of the present invention is to solve problems, associated with the surface quality, caused in the production of sheet products from a continuous thin cast strip-like slab.
A further object of the present invention is to provide an apparatus which enables a continuous thin cast strip-like slab to be stably rolled by means of a rolling machine connected directly with a casting machine.
A yet further object of the present invention is to provide a heat-treating furnace which can heat an as-rolled thin strip-like slab evenly in the widthwise direction.
A yet further object of the present invention is to provide an apparatus, for the production of a steel sheet, which can produce a steel sheet product without causing roping even when the sheet thickness has been changed during casting.